Planar light source

ABSTRACT

A surface illuminant comprises a light source  11  consisting of LEDs, a light guide plate  12  which is optically coupled to said light source for guiding the light to one side thereof and a reflective film  13  which is disposed on the other opposite side of said light guide plate. The light guide plate  12  is made of a material into which a wave length converting material  14  or a light storage material  15  such as fluorescent material is incorporated. A desired color light can be obtained by converting the wave length of the light from the light source  11  or light can be emitted by storing the light when the light source is turned off.

TECHNICAL FIELD

The present invention relates to a surface illuminant including lightemitting diodes (LEDs) and the like which are used as a light source andin particular to a surface illuminant which is capable of emitting adesired color light and is capable of reducing the electric powerconsumption.

BACKGROUND TECHNOLOGY

LED back light sources using a reflection plate have been widely used asa surface illuminant for compact display used for, for example, cellularphones.

FIG. 6 is an exploded perspective view showing a prior art surfaceilluminant which is exploded. In the drawing, a reference numeral 101denotes a light source comprising, for example, LEDs or CFL (coldfluorescent tubes); 102 denotes a light guide plate which is adjacent tosaid light source. The light guide plate 102 is formed of a white andtransparent plate made of polycarbonate or acrylic resin, etc. The lightguide plate 102 has a flat face from which light is emitted and areverse face on which a multiplicity of prism-like grooves in which thedistance between the grooves becomes narrower as it is more distant fromthe light source are formed as shown in the drawing by etching or sandblast process, so that the light from the light source will be reflectedon the faces of the grooves for uniformly emitting light from the entiresurface of the light guide plate 102. A reference 103 denotes a whitereflective film made of an appropriate synthetic resin such as polyester(PET), polycarbonate, which is bonded to the edge of the reverse side ofthe light guide plate 102 with a both-sided adhesive tape. The film 103reflects the light which has been transmitted through the faces of thegrooves having prism-like cross section for guiding the light to thelight guide plate 102 again so that it is emitted externally from thesurface of the light guide plate 102.

In the above-mentioned arrangement, when the light source 101 is lit,the light from the light source 101 is transmitted through the lightguide plate 102 and is then reflected on the prism-like faces formed onits reverse side or alternatively it is transmitted through theprism-like face and is reflected on the reflective film 103, and isincident upon the light guide plate 102, where it is mixed with thereflected light for substantially uniformly illuminating the entiresurface of the light guide plate 102.

At this time, the light from the light source is emitted as back lighthaving unchanged color since the light guide plate is colorless andtransparent.

It has been known that a problem occurs in which a surface illuminanthaving white and intermediate color can not be obtained since LED is notcapable of emitting white or intermediate color light when LEDs are usedas light source.

(1) In order to overcome this problem, JA-P-7-176794 proposes a surfaceilluminant comprising a fluorescent light scattering layer made of amixture of a fluorescent material with white powder for scatteringfluorescent light, which are applied on either one of the sides of alight guide plate in which light emitted from blue light LEDs issubjected to wave-length conversion by said fluorescent light scatteringlayer and a desired color including white color can be provided bychanging the fluorescent material.

(2) Alternatively, JP-A-8-7614 proposes a surface illuminant comprisinga scattering layer which is formed by applying white powders forscattering fluorescent light on either one of the sides of a light guideplate and further comprising a transparent film containing a fluorescentmaterial, which is disposed on the main face of the light guide plateopposite to said scattering layer, whereby desired colors includingwhite color can be provided.

However, it is inconvenient that the fluorescent scattering layer shouldbe applied (printed) on the light guide plate since the fluorescentmaterial is directly applied upon the light guide plate in the surfaceilluminant as set forth in paragraph (1). Since the color tone maychange depending upon the mixture ratio between the fluorescent materialand white powder for scattering fluorescent light, a desired color cannot be readily obtained.

In the surface illuminant as set forth in paragraph (2), it is alsonecessary to apply the fluorescent material and to provide a transparentfilm on which the fluorescent material is to be applied on the surfaceof the light guide plate in addition to the reflective plate provided onthe reverse side of the light guide plate. This type of surfaceilluminant is complicated in structure and its assembly requires a longtime and much labor.

Since the light in the light guide plate is terminated unless the lightsource is continuously lit in the above-mentioned both surfaceilluminants, it is necessary to constantly turn on the light source whenthe surface illuminant is used. This problem is not negligible in theportable electronic devices such as cellular phones from the view pointof power consumption since all the power is supplied from a batterycell.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome the above-mentionedproblems that the prior art surface illuminants encounter and to providemeans which is capable of emitting white or intermediate color with asimple structure in a surface illuminant using LED light sources andenables the surface illuminant to continuously emit light withoutcausing LEDs to continuously emit light.

The means which accomplishes the object will now be described.

[First Means]

The first means resides in a surface illuminant comprising a lightsource, a light guide plate which is optically coupled to said lightsource for emitting light incident from said light source through oneside thereof; and a reflective film which is disposed on the other sideof said light in which a wave length converting material is applied onsaid reflective film.

Since the surface illuminant of the first means includes the wave lengthconverting layer on the reflective film of the surface illuminant, thereflective film per se may be prior art white color reflective film.Accordingly, the surface illuminant of white and intermediate color canbe readily obtained. The surface illuminant of desired color can beobtained by merely changing the reflective film.

[Second Means]

The second means resides in a surface illuminant comprising a lightsource, a light guide plate which is optically coupled to said lightsource for emitting light incident from said light source through oneside thereof; and a reflective film which is disposed on the other sideof said light guide in which said reflective film comprises a lightstorage material.

Since the reflective film of the surface illuminant includes a lightstorage film in the surface illuminant of the second means, the surfaceilluminant is capable of continuously emitting light even if the lightsource is turned off for a predetermined period of time. The powerconsumption of the surface illuminant can be remarkably reduced.

[Third Means]

The third means resides in a surface illuminant comprising a lightsource, a light guide plate which is optically coupled to said lightsource for emitting light incident from said light source through oneside thereof; and a reflective film which is disposed on the other sideof said light guide in which said light guide plate is made of atransparent material in which a light storage material is blended.

Since the light guide plate into which the light storage material isincorporated is provided in the surface illuminant of the third means,the surface illuminant which is capable of storing light can be readilyobtained without specially treating the prepared light guide plate.Since the surface illuminant is capable of emitting light even if thelight source is turned off, the power consumption of the surfaceilluminant ca be remarkably reduced.

[Fourth Means]

The fourth means resides in a surface illuminant comprising a lightsource, a light guide plate which is optically coupled to said lightsource for emitting light incident from said light source through oneside thereof; and a reflective film which is disposed on the other sideof said light guide in which a light storage material is formed on thesurface of said light guide plate.

Since the light storage film is formed on the light guide plate in thesurface illuminant of the fourth means, the concentration of the lightstorage material, that is the amount of stored light can be relativelyeasily adjusted by controlling the amount of the applied light storagematerial.

[Fifth Means]

The fifth means resides in a surface illuminant of the fourth means inwhich the concentration of the light storage material in said lightstorage layer is increased from its one end adjacent to the light sourceto the other end thereof.

The illumination of the surface illuminant can be kept substantiallyuniform over the entire surface thereof by compensating for theattenuation of the light emitted to the light guide plate from the lightsource due to the fact that the concentration of the light storagematerial in the light guide plate is increased in accordance with thedistance from the light source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view showing the surface illuminant ofthe first and second embodiments of the present invention;

FIG. 2 is a view explaining a condition of driving the illuminant in thepresent invention;

FIG. 3 is an exploded perspective view showing the surface illuminant ofthe third embodiment of the present invention;

FIG. 4 is an exploded perspective view showing the surface illuminant ofthe fourth embodiment of the present invention;

FIG. 5 is an exploded perspective view showing the surface illuminant ofthe fifth embodiment of the present invention; and

FIG. 6 is an exploded perspective view showing a prior art surfaceilluminant.

BEST MODES FOR EMBODYING THE INVENTION

Embodiment of the surface illuminant of the present invention will nowbe described.

FIG. 1 shows first and second embodiments of the present invention.

FIRST EMBODIMENT

In FIG. 1, reference numerals 11, 12 and 13 denote a light source; alight guide plate made of a colorless transparent synthetic resin suchas polycarbonate or acrylic resin, which is formed on its reverse sidewith prism-like grooves; and a reflective film made of PET orpolycarbonate, respectively. These structures are substantiallyidentical with those of prior art which have been described withreference to FIG. 6.

In the surface illuminant, the light guide plate 12 is substantiallyrectangular in shape. The light source 11 is disposed adjacent to thelight guide plate 12 so that light is incident upon one end face of thelight guide plate 12 from, for example blue color LEDs. The reflectivefilm 13 is disposed to cover the reverse side of the light guide plate12. A wave length converting layer 14 is provided on the reflective film13.

The wave length converting layer 14 is formed by applying a solution ofa wave-length converting material such as a fluorescent material whichis excited on exposure to ultraviolet rays from the light source such asYAG for emitting fluorescent light. The solution contains the wavelength material, a resin such as acrylic resin and a proper solvent.Accordingly, when, for example, the blue color LEDs are used as thelight source 11, the ultraviolet rays from the blue color LEDs aresubjected to wave length conversion, so that fluorescent light isemitted.

Of course, it is possible to use the light guide plate 12 which isformed with convexes and concaves having different densities and sizesdepending upon the distance from the light source by embossing in lieuof the prism-like grooves on the reverse side thereof.

In order to obtain the white color light, a fluorescent material (whichis obtained by mixing equal amounts of red and green fluorescentpigments) which emits yellow color light on exposure to ultraviolet raysis used.

In the above-mentioned arrangement, when blue color LEDs are lit, partof light incident upon the light guide plate is transmitted through theprism-like face of the light guide plate and will impinge upon thereflective film 13. At this time, the wave length of the ultravioletrays in the transmitted light is converted with the fluorescent materialin the film so that the ultraviolet rays are converted into yellowreflected light. The reflected light is transmitted through theprism-like face again and enter the light guide plate, where it is mixedwith blue color from the LEDs to become white color light.

A surface illuminant which is capable of emitting critical and delicateintermediate color light which has been heretofore difficult to obtaincan be provided by appropriately selecting the fluorescent material 14and the wave length of the light source 11.

A surface illuminant which has an excellent light emission efficiencyeven if light having the same color as that of the light source can beobtained by providing the reflective film with a fluorescent materialwhich emits light having the same wave length as that of the lightsource 11. The power consumption of the surface illuminant can be madeless.

SECOND EMBODIMENT

The first embodiment in which the reflective film comprises thefluorescent material has been described. Now, a second embodiment of thesurface illuminant of the present invention will be described. Thesurface illuminant comprises a light storage material (a material whichabsorbs the light rays emitted from the light source for emitting lightfor a predetermined period of time after light rays from the lightsource disappear, for example, “N YAKOH” (trade name) manufactured byNemoto Kagaku Co. Ltd.) in lieu of the reflective film.

In the second embodiment, the reflective film made of a synthetic resinincludes a light storage material 15. The light storage material 15which is in the form of a solution including a resin as a binder and anappropriate solvent similarly to the case of said fluorescent materialis applied on the reflective film or alternatively the light storagematerial 15 is incorporated in the transparent (transmissive) reflectivefilm. If the light storage material is incorporated in the reflectivefilm, a reflective layer having white color is formed on the lower sideof the reflective film. If the light storage material is applied on thereflective film, the light storage material to be used specifically mayinclude, for example, “CHIKKO INK” and “N YAKOH CHIKKO INK” (tradenames) manufactured by Teikoku Ink Co. Ltd.

When the light source of, for example, blue light LEDs is lit in thearrangement of the second embodiment, part of the light which isincident on the light guide plate is transmitted through the prism likeface of the light guide plate and impinges on the reflective film 13. Atthis time, part of the impinged light is stored by the light storagematerial 15 of the reflective film. The stored light will be emitted fora predetermined period of time after turning off the light source sincethe surface illuminant of the present invention is capable of emittinglight for a predetermined period of time by the spontaneously emittedlight from the light storage material after the turning off of the lightsource in such a manner, the surface illuminant can continuously emitlight by repeating turning on and off the light source as shown in FIG.2. The power consumption can be remarkably reduced in comparison withthe prior art since it is not necessary to continuously turn on thelight source. The illumination of the surface illuminant can be keptuniform over the entire of the light emitting surface by increasing theamount of the applied light storing material 15 on the reflected filmdepending upon the distance from the light source to compensate for theattenuation of the light emitted into the light guide plate from thelight source.

THIRD EMBODIMENT

FIG. 3 shows a third embodiment of the present invention. In FIG. 3,reference numerals 31, 32 and 33 denote a light source; a light guideplate made of a colorless transparent synthetic resin such aspolycarbonate or acrylic resin, which is formed on its reverse sideprism-like grooves; and a reflective film made of PET or polycarbonate,respectively.

The light guide plate 32 is substantially rectangular in shape. Thelight source 31 is disposed adjacent to the light guide plate 32 so thatlight is incident upon one end face of the guide plate 32 from, forexample blue color LEDs. The reflective film 33 is disposed to cover thereverse side of the light guide plate 32.

The structure is substantially identical with that of the prior artshown in FIG. 6. In the third embodiment of the surface illuminant ofthe present invention, the light guide plate 32 is made of a colorlesstransparent synthetic resin such as polycarbonate or acrylic resin, inwhich a light storage material 34 is incorporated.

It is of course to use the light guide plate 32 which is formed withconvexes and concaves having different densities and sizes dependingupon the distance from the light source by embossing in lieu of theprism-like grooves on the reverse side thereof.

FOURTH EMBODIMENT

FIG. 4 shows a fourth embodiment of the present invention. In FIG. 4,reference numerals 41, 42 and 43 denote a light source; a light guideplate made of a colorless transparent synthetic resin such aspolycarbonate or acrylic resin, which is formed on its reverse sideprism-like grooves; and a reflective film made of PET or polycarbonate,respectively. Reference numerals 44 and 45 denote light storage materialand a light storage layer, respectively.

The fourth embodiment is substantially identical with the thirdembodiment in that the light guide plate 42 is made of a colorlesstransparent synthetic resin such as polycarbonate or acrylic resin,which is blended with a light storage material although it is differentfrom the third embodiment in that the light storage layer 45 is formedby applying the light storage material on the face side and/or reverseside of the light guide plate.

FIFTH EMBODIMENT

FIG. 5 shows a fifth embodiment of the present invention. In FIG. 5,reference numerals 51, 52 and 53 denote a light source; a light guideplate made of a colorless transparent synthetic resin such aspolycarbonate or acrylic resin, which is formed on its reverse sideprism-like grooves; and a reflective film made of PET or polycarbonate,respectively. A reference numeral 54 denotes light storage layer.

The fifth embodiment shows one example of detailed means for increasingthe concentration of the light storage layer from one end adjacent tothe light source to the other end so that the concentration of the lightstorage layer 54 of the light guide layer 52 is increased depending uponthe distance from the light source 51. In this case, a plurality ofstripes of ink each having different concentration of the light storagematerial which are applied from one end adjacent to the light source tothe other end when such an arrangement is adopted, the illumination ofthe surface illuminant can be kept uniform over the entire lightemitting surface by the light storage layer compensating for theattenuation of the light since the amount of the relative stored lightin the light guide plate is reversely proportional to the distance fromthe light source even if the light incident upon the light guide plateattenuates as it travels far from the light source.

Although the fourth and fifth embodiments in which the light guide plateis blended with the light storage material have been described, thepresent invention is not limited to those embodiments. The light guideplate which is not blended with the light storage material may be used.

If the light storage material is applied on the face of the light guideplate, a solvent of the light storage material is prepared by mixing itwith a resin binder and an appropriate solvent. The solvent is appliedon the face and reverse sides of the light guide plate. The lightstorage material to be used specifically may include, for example,“CHIKKO INK” and “N YAKOH CHIKKO INK” (trade names) manufactured byTeikoku Ink Co. Ltd.

When the light source 21, 31, 41, 51 is lit in the arrangement in FIGS.1, 3, 4, 5, part of the light which is incident on the light guide plateis stored in the light storage material 15 of the reflective film, thelight storage material 34, 44, 54 in the light guide plate and/or thelight storage layer 45 applied on the face and reverse sides of thelight storage plate. The stored light is emitted for a predeterminedperiod of time after turning off the light source since the surfaceilluminant of the present invention is capable of emitting light for apredetermined period of time after the turning off of the light source.

Since the surface illuminant of the present invention is capable ofemitting light for a certain period of time by the spontaneously emittedlight from the light storage material after turning off of the lightsource, the surface illuminant is capable of uniformly and continuouslyemitting light by repeating turning on or off of the light source asshown in FIG. 2. The power consumption can be remarkably reduced incomparison with the prior art since it is not necessary to continuouslyturn on the light source.

1. A surface illuminant comprising a light source, a light guide platehaving opposed sides and being optically coupled to said light sourcefor emitting light incident from said light source through one opposedside thereof, said light guide plate being formed with convexes andconcaves having different densities or sizes depending upon the distancefrom the light source on one side thereof, and a reflective film whichis disposed on the other opposed side of said light guide plate, whereinsaid reflective film comprises a light storage material incorporated inthe reflective film, wherein said storage material permits emission oflight for a period of time after light from said light sourcedisappears.
 2. A surface illuminant comprising a light source, a lightguide plate having opposed sides and being optically coupled to saidlight source for emitting light incident from said light source throughone opposed side thereof, said light guide plate being formed withconvexes and concaves having different densities or sizes depending uponthe distance from the light source on one side thereof, and a reflectivefilm which is disposed on the other opposed side of said light guideplate, wherein said light guide plate is made of a transparent materialin which a light storage material is blended.
 3. A surface illuminantcomprising a light source, a light guide plate having opposed sides andbeing optically coupled to said light source for emitting light incidentfrom said light source through one opposed side thereof, said lightguide plate being formed with convexes and concaves having differentdensities or sizes depending upon the distance from the light source onone side thereof, and a reflective film which is disposed on the otheropposed side of said light guide plate, wherein a light storage film isformed on the surface of said light guide plate.
 4. A surface illuminantas set forth in claim 3, wherein the concentration of the light storagematerial in said light storage layer is increased from its one endadjacent to the light source to the other end thereof.
 5. A surfaceilluminant comprising a light source, a light guide plate having opposedsides which is optically coupled to said light source for emitting lightincident from said light source through one opposed side thereof, and areflective film which is disposed on the other opposed side of saidlight guide plate, wherein a light storage film is formed on the surfaceof said light guide plate, and wherein the concentration of the lightstorage material in said light storage layer is increased from its oneend adjacent to the light source to the other end thereof.